Atom-based electromagnetic field sensing element and measurement system

Abstract

Methods and apparatus for sensing or measuring an electromagnetic field. The method entails excitation into a distribution of Rydberg states of atoms of a gas occupying a test volume coextensive with the electromagnetic field. Transmission along a path traversing the test volume of at least one probe beam of electromagnetic radiation is measured at one or more frequencies overlapping a spectral feature, and a physical characteristic of the electromagnetic field is derived on the basis of variation of the spectral feature. In various embodiments, the electromagnetic field may be place in interferometric relation with another electromagnetic field. Time-varying electric field amplitude, frequency, phase and noise spectral distribution may be measured, and thus AM and FM modulated fields, as well as magnetic fields of about 1 Tesla. The apparatus for measuring the electromagnetic field may be unilaterally coupled to a probe field and detector or array of detectors.

We claim: 
     
       1. A method for sensing or measuring a first electromagnetic field, the method comprising:
 a. exciting, into a distribution of Rydberg states, atoms and/or molecules of a gas occupying a test volume coextensive, at least in part, with the first electromagnetic field; 
 b. structuring the first electromagnetic field by placing it in interferometric relationship with at least one other electromagnetic field; 
 c. measuring transmission, along a path traversing the test volume, of at least one probe beam of electromagnetic radiation; and 
 d. on the basis, at least, of the measured transmission, deriving a physical characteristic of the first electromagnetic field. 
 
     
     
       2. A method in accordance with  claim 1 , wherein the gas is an atomic or molecular vapor. 
     
     
       3. A method in accordance with  claim 2 , wherein atoms of the atomic vapor are chosen from a group of atoms including rubidium, cesium, alkali, and alkali earth atoms. 
     
     
       4. The method in accordance with  claim 2 , wherein the distribution of Rydberg states is comprised of one or more Rydberg states. 
     
     
       5. A method in accordance with  claim 1 , wherein the step of exciting atoms into a distribution of Rydberg states comprises optically exciting the atoms into a distribution of Rydberg states. 
     
     
       6. A method in accordance with  claim 1 , wherein the step of exciting atoms into a distribution of Rydberg states comprises at least one of electromagnetically induced transparency and electromagnetically induced absorption. 
     
     
       7. A method in accordance with  claim 1 , wherein a physical characteristic of the first electromagnetic field is based, at least, on a change in a spectral feature of the atomic gas, and wherein the change in the spectral feature includes Autler-Townes splitting. 
     
     
       8. A method in accordance with  claim 1 , wherein the physical characteristic of the first electromagnetic field is field amplitude. 
     
     
       9. A method in accordance with  claim 1 , wherein the physical characteristic of the first electromagnetic field is its phase relative to a fiducial phase. 
     
     
       10. A method in accordance with  claim 1 , wherein;
 the first electromagnetic field is monochromatic, and 
 the physical characteristic of the first, monochromatic electromagnetic field is its phase relative to a fiduciary phase of an external RF field that modulates one of the other electromagnetic fields. 
 
     
     
       11. A method in accordance with  claim 1 , wherein structuring the first electromagnetic field comprises superimposing an additional static or radio-frequency field to place the first electromagnetic field into resonance with an atomic transition. 
     
     
       12. A method in accordance with  claim 1 , wherein structuring the first electromagnetic field includes a modulating the first electromagnetic field prior to the step of measuring. 
     
     
       13. A method in accordance with  claim 12 , wherein modulating is at least one of frequency-, amplitude-, and step-modulation. 
     
     
       14. A method in accordance with  claim 1 , wherein the first electromagnetic field is amplitude, frequency, polarization, or phase modulated. 
     
     
       15. A method in accordance with  claim 1 , wherein the physical characteristic of the first electromagnetic field is one of its time-varying electric field amplitude, frequency, polarization, or phase. 
     
     
       16. A method in accordance with  claim 1 , wherein the at least one other electromagnetic field is introduced by an external RF reference wave. 
     
     
       17. A method in accordance with  claim 1 , wherein the at least one other electromagnetic field is introduced by electrodes integrated into the test volume or cavity structures. 
     
     
       18. A method in accordance with  claim 1 , wherein an external reference wave is introduced at the location of the atoms and/or molecules. 
     
     
       19. A method in accordance with  claim 1 , wherein the physical characteristic is the electric-field sum of the first electromagnetic field and a reference field. 
     
     
       20. The method in accordance with  claim 19 , wherein the magnitude of the electric-field sum of the first electromagnetic field and the reference field depends on a phase difference between the reference field and the first electromagnetic field. 
     
     
       21. The method in accordance with  claim 19 , wherein the physical characteristic is a beat frequency resulting from the electric-field sum of the first electromagnetic field and the reference field. 
     
     
       22. The method in accordance with  claim 1 , wherein the physical characteristic is a beat frequency resulting from structuring the first electromagnetic field by placing it in interferometric relationship with the at least one other electromagnetic field. 
     
     
       23. The method in accordance with  claim 1 , wherein the physical characteristic is a laser induced fluorescence. 
     
     
       24. A sensor for at least one of detecting and measuring a parameter characterizing a first electromagnetic field, the sensor comprising:
 a. an excitation source; 
 b. an enclosure containing a gas of atoms and/or molecules of which at least a subset may be excited by the excitation source into a distribution of Rydberg states; 
 c. a detector, disposed to detect a transmission of a probe beam after traversal of the gas by the probe beam; and 
 d. a processor configured to derive a parameter characterizing the first electromagnetic field based on the measured transmission of the probe beam. 
 
     
     
       25. A sensor in accordance with  claim 24 , wherein the transmission is sensitive to a phase characterizing the first electromagnetic field. 
     
     
       26. A sensor in accordance with  claim 24 , wherein the transmission is sensitive to a phase of transitions between Rydberg states characterized by a specified Rabi frequency. 
     
     
       27. A sensor in accordance with  claim 24 , wherein the parameter varies continuously in time. 
     
     
       28. A sensor in accordance with  claim 24 , wherein the gas includes a molecular vapor. 
     
     
       29. A sensor in accordance with  claim 24 , wherein the excitation source is specially configured to induce transitions between Rydberg states at a specified Rabi frequency. 
     
     
       30. A sensor in accordance with  claim 24 , wherein the parameter of the first electromagnetic field is the propagation direction. 
     
     
       31. A sensor in accordance with  claim 24 , wherein the parameter of the first electromagnetic field is at least one of an amplitude, frequency, phase, or polarization. 
     
     
       32. A sensor in accordance with  claim 24 , wherein the parameter of the first electromagnetic field is a modulation of at least one of amplitude, frequency, phase, or polarization. 
     
     
       33. A sensor in accordance with  claim 24 , wherein the parameter of the first electromagnetic field is a phase of the first electromagnetic field relative to a fiduciary phase. 
     
     
       34. A sensor in accordance with  claim 24 , further comprising a second electromagnetic field placed into interferometric relation with the first electromagnetic field. 
     
     
       35. A sensor in accordance with  claim 34 , wherein a phase of the second electromagnetic field is introduced by an external RF reference source. 
     
     
       36. A sensor in accordance with  claim 24 , wherein a phase of the first electromagnetic field is relative to an RF modulated optical beam. 
     
     
       37. A sensor in accordance with  claim 24 , wherein a phase of the first electromagnetic field is relative to an external RF reference wave. 
     
     
       38. A sensor in accordance with  claim 24 , further comprising an RF reference wave. 
     
     
       39. A sensor in accordance with  claim 24 , wherein the parameter of the first electromagnetic field is the electric-field sum of the first electromagnetic field and a reference field. 
     
     
       40. The sensor in accordance with  claim 39 , wherein the magnitude of the electric-field sum of the first electromagnetic field and the reference field depends on a phase difference between the reference field and the first electromagnetic field. 
     
     
       41. The sensor in accordance with  claim 39 , wherein the parameter of the transmission is sensitive to a beat frequency resulting from the electric-field sum of the first electromagnetic field and the reference field. 
     
     
       42. The sensor in accordance with  claim 24 , wherein the transmission is sensitive to a beat frequency resulting from structuring the first electromagnetic field by placing it in interferometric relationship with at least one other electromagnetic field. 
     
     
       43. The sensor in accordance with  claim 24 , wherein the transmission is sensitive to a laser induced fluorescence. 
     
     
       44. The sensor in accordance with  claim 24 , wherein the distribution of Rydberg states is comprised of one or more Rydberg states. 
     
     
       45. A sensor in accordance with  claim 24 , wherein the parameter of the first electromagnetic field includes characteristics of a source of the first electromagnetic field.